1. Field of the Invention
The present invention relates to a hard sintered body for a tool employing cubic boron nitride (hereinafter referred to as cBN).
2. Description of the Background Art
In general, cBN is widely known as the hardest material following diamond. Sintered bodies of cBN, having lower reactivity with ferrous materials as compared with diamond, are applied to various cutting tools.
Japanese Patent Laying-Open No. 53-77811 (1978) discloses examples of such cBN sintered bodies. This Patent Publication discloses a sintered body consisting of a material containing 80 to 40% of cBN in volume ratio and a rest or remainder mainly composed of a carbide, a nitride, a boride or a silicide of a transition metal belonging to the group IVa, VA or VIa of the periodic table or a mixture or a mutual solid solution compound thereof, and a sintered body prepared by adding Al and/or Si to this material.
However, a tool prepared from either sintered body disclosed in Japanese Patent Laying-Open No. 53-77811 is remarkably worn or abruptly chipped when used for high-speed working of hardened steel, for example.
The tool is worn conceivably because the temperature of its cutting edge is increased during high-speed cutting of hardened steel which thermally deteriorates binders or cause cBN particles to drop out Further, the tool is abruptly chipped conceivably because the binders are thermally deteriorated and reduced in holding power for the cBN particles.